Recently, equipment and structures such as ships, bridges, pipelines, or offshore structures have become bigger to provide value added products or services. In addition, many skyscrapers and infrastructure facilities are being constructed, and thus, demand for heavy equipment and construction machinery is also increasing.
Even a single accident or a relatively small accident involving such large structures may cause fatal environmental problems, casualties, and property loss. Thus, steels having ultrahigh-strength, very high thicknesses, and high impact toughness are used for such structures, and along with this, secure and effective welding techniques are required for such steels. Particularly, the impact toughness of weld zones of large welded structures may be the most important factor to be considered for stability or safety.
Practically, the productivity of welding may have to be considered prior to guaranteeing the impact toughness of weld zones, and in this regard, gas metal arc welding (GMAW) enabling automatic robot welding is widely used. GMAW is usually performed with a heat input of about 20 kJ/cm.
During a welding process, a weld pool is formed as a welding wire is melted and diluted with material from the base metal, and a welded joint having a coarse columnar microstructure may be formed as the weld pool solidifies. Such a microstructure of a welded joint may vary according to a welding material or the amount of heat input during a welding process. In welded joints formed as described above, phases such as coarse grain boundary ferrite, widmanstatten ferrite, martensite and a martensite-austenite (M-A) constituent are present along coarse austenite grain boundaries. Thus, the impact toughness of welded structures may be lowest at welded joints.
Therefore, it is required to improve the impact toughness of welded joints by controlling the microstructure of the welded joints, so as to guarantee the stability of welded structures.
To this end, Patent Document 1 discloses a technique of guaranteeing the stability of a welded structure by adjusting the composition of a welding material. However, according to the disclosed technique, factors such as the microstructure or grain size of a welded joint are not directly controlled, and thus it is difficult to sufficiently improve the toughness of a welded joint formed using such a welding material.
(Patent Document 1) Japanese Patent Application Laid-open Publication No. H11-170085